Pointing devices have been used with computers and other types of electronic systems for many years. A computer mouse is one example of a pointing device. With a mechanical mouse, a ball rolls over a surface as the mouse is moved. Interior to the mouse are wheels that contact the ball and convert its rotation into electrical signals representing orthogonal components of motion.
Another type of pointing device is an optical mouse. As an optical mouse moves over a surface, light emitted from a light source within the mouse reflects off the surface and is detected by a motion sensor positioned within the mouse. The motion sensor typically includes a camera that captures images of the surface. The motion sensor analyzes a sequence of images to determine the speed and distance the mouse has moved across the surface.
A basic optical mouse needs particular surface properties in order to accurately determine the motion of the mouse. When a surface is formed from a smooth material such as glass, or includes a reflective material such as a mirror, the absence of surface features in the surface means the motion sensor is unable to acquire the amount of data needed to determine the motion of the mouse.
Alternative optical techniques have been investigated to address this problem. Speckle and other interferometric techniques measure variations in the surface on the scale of the wavelength of light. These small variations create interference patterns that can, in theory, be used to determine motion. Unfortunately, the surface variations in clean, undamaged glass are not sufficient to create strong optical signals, making it difficult for the motion sensor to determine the motion of the optical mouse.